A multi-frequency radio study of Cygnus A

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Abstract
New observations of Cygnus A made with the Cambridge 5-km radio telescope at 2.7, 5, 15.4 and 31.4 GHz are presented. The increased dynamic range over earlier 5-km maps at these frequencies has enabled us to map spectral variations over a large proportion of the source. The spectra are found to steepen away from the hotspots, with the spectral index reaching a maximum of about 2 in the most extended structure. We have interpreted these variations as due to synchrotron ageing and thus have estimated the speed of advance of the hotspots. For each lobe, we find the speed of the material relative to the hotspot is ∼ 0.1 c. If we assume a model including back-flow of material from the hotspots this is reduced to 0.05 c. Despite having polarization maps at three frequencies we are unable to determine unambiguously the distribution of rotation measure across the source, although our results are in broad agreement with earlier work.

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